The development of high-quality polymer semiconductor heterostructures is crucially needed to the further improve the performance of polymer semiconductor devices. In light-emitting diodes for example, heterostructures are central for efficient charge-carrier injection and confinement, and also for control over their recombination, and the fate of the excitons generated. However this important goal has been hindered in the past by the lack of a sufficiently general cross-linking system that is suitable for cross-linking polymer semiconductors without degrading their charge-carrier transport and exciton properties.
Various methods involving the use of specific cross-linking chemistry have been proposed such as epoxy or oxetane ring-opening under acid-catalysis, or cycloaddition reactions. In a specific cross-linking reaction, two functional groups react together in the presence of light or heat to give a cross-link.
However these specific cross-linking chemistries have several characteristics that may not be advantageous. First, they require a very high concentration of the cross-linking moieties to be present, typically well above 10 mol % of a typical polymer repeat unit, so that a high enough local concentration for the bimolecular reaction may take place since the two reacting moieties have to come into contact. Such high concentrations of the cross-linker moieties may alter the desired morphological characteristics of the polymer. Second, a significant fraction of these cross-linking moieties are unfortunately stranded and so do not form cross-links, because they cannot find a cross-linking partner in the time they are active. These cannot be removed subsequently and thus give rise to an electronically significant concentration traps for charges, particularly electrons, and for excitons.
To overcome these two limitations, the use of non-specific cross-linking chemistry through fluorinated phenyl azides has been proposed (WO 2004/100282). Fluorinated phenyl azides can be photolysed to nitrenes when exposed to 254 nm (i.e., deep UV light) that insert into unactivated CH bonds. However, some loss of performance due to electron trapping and exciton quenching occurs particularly at high cross-linker concentrations.
There is therefore a need for an improved cross-linking moiety which may be suitable for cross-linking polymer semiconductors.